Super PACs are able to out-raise traditional campaigns because the committees can accept donations of unlimited amounts from wealthy individuals, corporations or unions. But donors to candidates can only give $2,500 per candidate, per election.

In some cases, a super PAC can be little more than an extension of a single person. The pro-Gingrich super PAC, Winning Our Future, raised $11 million in January—of which $10 million came from billionaire casino owner Sheldon Adelson and his wife, Miriam.

Of the $2.4 million collected by the pro-Ron Paul super PAC Endorse Liberty, $1.7 million came from PayPal co-founder Peter Thiel. The pro-Rick Santorum super PAC Red, White and Blue Fund collected $2.1 million in January, but almost all of it came from two people: $1 million from oil equipment executive William J. Doré and $669,000 from Foster Friess.

By comparison, Mitt Romney’s super PAC donors are positively diverse—it took forty of them to raise most of Restore Our Future’s $6.6 million.